The present invention relates to power-line-carrier communications systems and, more particularly to a power-line-carrier communications system capable of shifting operating frequency to avoid power-line interference.
It has long been known to communicate information utilizing a power-line as the communication medium. In such a power-line-carrier (PLC) communications system, the desired signal may be interfered with by undesired signals, collectively labeled "noise", upon the power-line medium. Typically, the frequency spectrum of such noise signals, which may be of narrowband noise nature generated by appliances and other communications systems connected to the power-line, is not known at the time of system installation. Even if known, this noise spectrum may change with time, as new power-line loads and power-line-utilizing devices are added and existing loads and devices change characteristics with changes in the applicable environment. There is thus always a risk that an interferring signal will be present at the PLC system operating frequency and will greatly degrade the reliability of PLC communications.
Hitherto, two distinct alternative approaches have been utilized in an attempt to increase the signal-to-noise ratio of the PLC signal with respect to noise signals. The first approach is the so-called "narrowest band" approach, in which the desired baseband signal is communicated over the narrowest possible frequency bandwidth, so as to remove as much of the potential interference as possible by narrow bandpass filtering. This approach is often adequate if the interference encountered is not concentrated in the band of communication frequencies actually in use. The remaining approach is the so-called "spread spectrum" approach, wherein the baseband signal is deliberately mapped over a much greater frequency band than would normally be required for transmission thereof. This spread spectrum approach is advantageous if the interference across the band is uncorrelated to the PLC communication signal. However, spread spectrum PLC systems are susceptible to correlated broadband interference and carriers, to an extent dependent upon the manner in which the spread spectrum modulation is implemented and the nature of the interfering noise signal. Further, a spread-spectrum PLC system frequently does not provide filtering before the active receiving circuitry, whereby a strong interfering carrier (having a frequency located in any portion of the spread-spectrum frequency band) can completely disrupt PLC communications by saturation of the PLC receiver input circuitry. Accordingly, it is highly desirable to improve the reliability of power-line communications in the presence of man-made interference and other noise.